1. Field of the Invention
The present invention relates to a color thermal printer, and more particularly to a color thermal printer in which a color registration error can be prevented even at the time of variation of a printing load.
2. Description of the Prior Arts
As a color thermal printer, there has been known a color thermal printer in which a full color image is printed on a color thermal recording paper in a three-color frame sequential fashion. The color thermal recording paper includes yellow, magenta, and cyan thermosensitive coloring layers overlaid on a support in sequence. The color thermal recording paper is fed by feeder roller pairs constituted of a capstan roller and a pinch roller. While feeding the recording paper, each heating element of a thermal head is caused to generate heat based on image data, and thereafter one of three primary color images is printed line by line.
The value integrated a coefficient of dynamic friction between a thermal head and a surface of the recording paper to pressing force of the thermal head acts as the printing load. Heat energy of the color thermal recording paper is different by each primary color to be recorded. If the heat energy to be applied to the color thermal recording paper is increased, a protective layer which covers outside of the yellow thermosensitive coloring layer is softened, so that the coefficient of dynamic friction is lowered. If the coefficient of dynamic friction is lowered, feeding speed of the color thermal recording paper becomes fast because the printing load is also lowered. Since the feeding speed of the recording paper is changed by the color to be recorded, the color registration error, density unevenness, and so forth are created.
Therefore, there has been known a color thermal printer in which the paper feeding speed of the color thermal recording paper is kept constant by controlling of the rotation speed of a feeding motor in response to the printing load calculated based on the image data, for example, as disclosed in Japanese Patent Laid-Open Publication No. 2002-029078. In addition, it is known to adjust one line print cycle based on the printing load calculated based on the image data, for example, as disclosed in U.S. Pat. No. 6,108,019 (corresponding to Japanese Patent Laid-Open Publication No. H11-058806).
However, the coefficient of dynamic friction in the color thermal recording paper is changed by humidity, contamination of the thermal head, and type of the color thermal recording paper. Therefore, the color registration error and the density unevenness cannot be prevented effectively only by consideration of the printing load based on the image data.
An object of the present invention is to provide a color thermal printer for printing an image in consideration of variation of a printing load created by different factors except for image data.
Another object of the present invention is to provide a color thermal printer in which it is possible to prevent color registration error and density unevenness from occurring.
In order to achieve the above and other objects, a color thermal printer of the present invention includes correction means for setting a correction coefficient, a determination device for calculating a printing load of each line, which is corrected by the correction coefficient, and a motor controller for controlling rotation speed of a feeding motor according to the printing load of each line.
The correction means includes at least one of a paper type correction unit, a humidity correction unit, and a sheet number correction unit. The paper type correction unit determines a paper type correction coefficient based on a type of a recording paper. The humidity correction unit determines a humidity correction coefficient based on the humidity. The sheet number correction unit determines a sheet number correction coefficient based on accumulative sheet number. The correction coefficient is determined based on the paper type correction coefficient, the humidity correction coefficient, and the sheet number correction coefficient.
The determination device is operated for calculating the printing load of each line based on image data, pressing force of a thermal head, and the correction coefficient. The motor controller is operated for changing the rotation speed of the feeding motor according to the printing load of each line and feeding the recording paper at regular speed without being affected by the printing load.
According to the preferred embodiment of the present invention, the determination device is constituted of a first converter for converting the image data of one line to the heat energy of each heating element, a second converter for converting the heat energy to each coefficient of dynamic friction, and a printing load calculator for calculating the printing load of one line based on each coefficient of dynamic friction, the correction coefficient, and the pressing force.
According to another preferred embodiment of the present invention, the one-line printing cyclic process is changed in response to the printing load instead of controlling the rotation speed of the feeding motor.
According to the present invention, the printing load, which is obtained on the basis of the image data, is corrected in accordance with different factors, such as the humidity, the sheet number, the paper type, and so forth, so that it is possible to obtain more precise printing load, and to deter the occurrence of the color registration error and the density unevenness in an effective manner.